1. Field of the Invention
The present invention relates to an SMD-type piezoelectric vibrator in which a piezoelectric vibrating reed is sealed in a cavity formed between two bonded boards, a manufacturing method of the piezoelectric vibrator, and an oscillator, an electronic device, and a radio-controlled clock each having the piezoelectric vibrator.
2. Description of the Related Art
Recently, a piezoelectric vibrator using a piezoelectric vibrating reed made of a piezoelectric material such as quartz crystal is used in a cellular phone and a portable information terminal as the time source, the timing source of a control signal, a reference signal source, and the like. As the piezoelectric vibrating reed, a turning-fork type piezoelectric vibrating reed having a pair of vibrating arms is used.
As the piezoelectric vibrator of this type, an SMD (Surface Mount Device)-type piezoelectric vibrator is known.
As an SMD-type piezoelectric vibrator 200, there is proposed one as shown in FIGS. 14 and 15 in which a package 209 is formed by a base board 201 and a lid board 202, and a piezoelectric vibrating reed 203 is accommodated in a cavity C formed inside the package 209. The base board 201 and the lid board 202 are bonded to each other by an anodic bonding using a bonding film 207 which is disposed between both boards.
Meanwhile, in general piezoelectric vibrators, it is preferable to suppress an equivalent resistance value (effective resistance value Re) to a low value. Since a piezoelectric vibrator having a low equivalent resistance value is capable of vibrating a piezoelectric vibrating reed with a low power, a piezoelectric vibrator having high energy efficiency can be achieved.
As a typical method of suppressing the equivalent resistance value, as shown in FIGS. 14 and 15, there is known a method of creating a near-perfect vacuum in the sealed cavity C of the piezoelectric vibrating reed 203 so as to decrease a series resonance resistance value (R1) which is proportional to the equivalent resistance value. Moreover, as a method of creating a near-perfect vacuum in the cavity C, JP-A-2003-142976 discloses a method (gettering method) of sealing a gettering material 220 made, for example, of aluminum in the cavity C and activating the gettering material 220 with laser irradiation from the outside. According to this method, since oxygen generated at the time of anodic bonding can be absorbed by the activated gettering material 220, it is possible to create a near-perfect vacuum in the cavity C.
Moreover, in general, after the gettering step, a step (fine tuning step) of irradiating a weight metal film 211 formed at the tip end of a vibrating arm 210 with a laser beam to trim the weight metal film 211, thus finely tuning the frequency of the piezoelectric vibrating reed 203 is performed. By performing this fine tuning step, the frequency of the piezoelectric vibrating reed 203 can be made to fall within the predetermined range of the nominal frequency.
However, in the manufacturing method of the piezoelectric vibrator according to the related art, since the fine tuning step is performed after the gettering step so that the degree of vacuum in the cavity and the frequency of the piezoelectric vibrating reed are adjusted respectively, the adjustment takes lots of time, thus deteriorating manufacturing efficiency. Moreover, the formation of the gettering material necessary for gettering takes time, and the manufacturing efficiency is deteriorated further.